 Mac-1

Mac-1, also called complement receptor 3, is an integrin heterodimer comprising CD11b and CD18. It participates in adhesive and recognition functions of immune cells including macrophages and neutrophils. Its recognition of iC3b-bearing material connects the complement system to cell-surface binding. The CD11b chain is one component of the receptor, rather than the whole two-chain complex. ^[ref]

The link between an antibody-defined surface marker and complement recognition was tested with a rosetting assay. Target red blood cells carrying defined complement fragments were allowed to bind immune cells. Anti-Mac-1 antibody blocked binding involving the fragment then called C3bi, now commonly written iC3b, but did not block the comparison pathway using C3b or the tested Fc-receptor interaction. Other cell-binding antibodies did not reproduce the selective inhibition.^[ref]

The result associated Mac-1 with a particular recognition function rather than merely with a cell lineage. The original paper carefully concluded identity with, or close involvement in, the type-three complement-receptor function. Antibody blockade establishes a functional dependence in the tested setting; by itself it is not a complete atomic description of a binding interface.

Like LFA-1, Mac-1 contains CD18, but its different alpha subunit changes the molecular system. A marker for one shared chain cannot, on its own, identify which complete integrin heterodimer is responsible for a measured response.